Fiber-reinforced composite materials (hereafter also abbreviated as FRP) are lightweight, while offering good strength and high rigidity, and are consequently widely used in a variety of applications from sports and leisure through to industrial applications such as vehicles and aircraft. In recent years, with the fall in the cost of carbon fiber, the use of carbon fiber reinforced composite materials (hereafter abbreviated as CFRP), which are even more lightweight and offer even higher levels of strength and rigidity, within industrial applications has also become more widespread.
Amongst these potential industrial applications, CFRPs used for structural members within train bodies and aircraft frames are typically produced by autoclave molding, using an intermediate material known as a prepreg. The reason for this preference is that by conducting the molding under high pressure using an autoclave, not only can the occurrence of voids within the molded product be reduced, enabling the strength of the molded product to meet expectations, but the occurrence of surface pinholes can also be suppressed, enabling the production of a molded product with a favorable external appearance.
However, autoclave facilities are extremely expensive, which not only acts as a large barrier to new entrants, but also means that once autoclave facilities are purchased, the size of the molded products is restricted by the size of the autoclave, meaning the production of larger products is effectively impossible.
In order to avoid these problems, the development of autoclave-free, low cost molding is also progressing, and representative examples of such molding include oven molding under either vacuum conditions or normal atmospheric conditions (also known as vacuum bag molding). Oven molding does not require the application of pressure, meaning the molding can be conducted without the need for a proper pressure-resistant vessel such as an autoclave, and molding can be conducted simply with a furnace for raising the temperature. Molding can also be conducted with a simple device comprising an adiabatic board and a hot air heater. However, because these processes do not involve the application of pressure, residual voids tend to remain within the molded product, the strength of the molded product is inferior to that of a molded product produced in an autoclave, and pinhole formation is also a problem.
In recent years, a variety of measures for overcoming these problems have been proposed. For example, WO 00/27632 discloses technology relating to materials comprising a resin layer and a reinforcing fiber layer, which display minimal void generation, and enable the production of molded products with extremely clean surfaces, even when used with oven molding. However, with this technology, almost all of the resin is impregnated during molding, and depending on the molding conditions, portions of the resin that display unsatisfactory impregnation can occur, leading to the occurrence of internal voids and surface pinholes. Furthermore, because the surface is almost free from resin and is extremely dry, workability problems such as difficulty in bonding the product to the molding die can also be a concern.